1. Technical Field of the Invention
This invention relates generally to the field of the removal of sulfur from hydrocarbonaceous products. Specifically, the invention relates to a heterogeneous catalyst and process for the oxidative desulfurization of hydrocarbonaceous oil.
2. Description of the Prior Art
In the petroleum industry, it is common for hydrocarbonaceous oils, particularly middle distillate petroleum fuels, to contain sulfur species. Increasing concerns regarding pollutants present in the atmosphere have led to a desire to decrease the sulfur content of fuels used in engines, as engines and vehicles utilizing fuels which contain sulfur can produce emissions of nitrogen oxide, sulfur oxide and particulate matter. Government regulations have become more stringent in recent years with respect to allowable levels of the potentially harmful emissions.
Traditionally, thiophenic sulfur species contained in hydrocarbonaceous feedstock are removed by catalytic hydrodesulfurization at high temperature and high pressure in the presence of hydrogen. Typical hydrodesulfurization catalysts generally include cobalt-molybdenum sulfide, nickel-molybdenum sulfide or nickel tungsten sulfide, typically supported on alumina based materials. Hydrodesulfurization catalysts are commercially available and are generally known in the art.
There are however deficiencies in the removal of thiophenic species from hydrocarbonaceous sources by catalytic hydrodesulfurization.
Although reliable in the removal of thiophenic sulfur species, and capable of achieving relatively low sulfur content, catalytic hydrodesulfurization requires a low flow rate, high hydrogen pressure, high hydrogen consumption and high reaction temperatures to achieve sulfur contents of less than 10 ppm. Additionally, in certain applications, a sulfur content of near zero, or less than 1 ppm, may be desired. These specifications are difficult to achieve with present methods.
It is known that during desulfurization of a hydrocarbon stream, as the severity of the desulfurization conditions is increased, the level of residual sulfur containing hydrocarbons decreases. This is because certain sulfur compounds are more susceptible to catalytic desulfurization under certain reaction conditions than are others. Not wishing to be bound to any theory, it is believed that the location of the sulfur atom plays an important role in determining if the sulfur atom will more readily react with the catalytic site. If the sulfur atom is sterically hindered, it is believed that more severe reaction conditions must be employed in order to react and remove the sulfur atom from the hydrocarbonaceous compound. Additionally, it is believed that the nitrogen compounds that are present in the hydrocarbonaceous stream are strong inhibitors against hydrodesulfurization of refractory sulfur species.
Sulfur is found in refinery streams in a number of different forms including aliphatic and aromatic sulfur compounds, but tends to be concentrated in the higher boiling fractions. Generally, the sulfur compounds are present in the form of thiophenes and aromatic heterocyclic compounds such as benzothiophenes and dibenzothiophenes. As noted previously, conventional hydrodesulfurization techniques are capable of removing most sulfur compounds, particularly the lower molecular weight species including the aliphatic sulfur materials, thiophenes and dibenzothiophenes. However, certain sulfur compounds, such as sterically hindered compounds, are more difficult to remove than others. For example, substituted dibenzothiophenes are generally less amenable to hydrodesulfurization than dibenzothiophene itself, likely because of the sterically hindered sulfur atom. This effect varies according to the extent and type of substitution in dibenzothiophene.
Conversion of dibenzothiophenes can require high hydrogen partial pressures and recycling rates, low space velocity and high temperatures, implying a significant increase in the capacity of the hydrogen recycling system, an increase in the reactor bed size, an increase in the operating pressure, a decrease in cycle length, or combinations thereof. Conducting the desulfurization at such severe reaction conditions generally results in relatively short catalyst life times.
Therefore, improved methods for the desulfurization of hydrocarbonaceous streams that include refractory sulfur compounds are needed. More specifically, methods for the desulfurization and production of hydrocarbonaceous streams containing less than 10 ppm sulfur, or more preferably, less than 5 ppm sulfur, are needed.